1. Field of the Invention
The present invention relates to an information detection apparatus, and more particularly to an information detection apparatus as represented by an encoder used for a displacement information measurement apparatus in measuring dimension, distance or velocity such as measurement control which needs a resolution power in the order of atom (several .ANG.).
2. Related Background Art
In the past, the encoder of this type comprises a reference scale having information on position or angle, and detection means relatively movable with the reference scale to detect information on position or angle. Such an encoder is classified into several types depending on the reference scale and the detection means, such as an optical encoder, a magnetic encoder and an electrostatic encoder.
As an encoder having a resolution power in the order of atom, a parallel displacement detection apparatus is disclosed in Japanese Laid-Open Patent Application No. 62-209302. It uses a basic principle of a scanning tunneling microscope disclosed in U.S. Pat. No. 4,343,993 which permits the observation of information on a sample surface at the atom resolution power.
In the past, such an encoder comprises a scale which is a reference to length and a probe arranged in the vicinity of the scale. Information derived from a tunneling current which flows between the reference scale having a drive mechanism and the probe is processed and encoded.
The probe which detects the tunneling current of the encoder is usually manufactured by a well-known electrolytic polishing method to form a sharp needle. Alternatively, machine polishing may be used.
However, the function of the probe which has the sharpness in the order of atom to detect the tunneling current is a heart of the encoder and the performance of the probe directly affects the performance of the encoder. In order to control and detect the tunneling current in the order of pA or nA which flows between the reference scale and the probe, it is necessary to set the distance between the reference scale and the probe to a very small distance such as several nm. Thus, when acoustic vibration or floor vibration occurs, the probe may contact the reference scale and the tip end of the probe may be broken so that it may no longer have the atom resolution power and permit the measurement in the order of atom.